You may touch upon any topic in your letter really, as long as you DO NOT write about politics, religion, death, or killing. Remember to put your return address on the back of the envelope.
The most common type of letters to veterans is the standard "Thank you for your service" letter. An example of one such letter is below:
Dear Service Member,
My name is Lord Jeff and I am a junior at Amherst College, a small liberal arts institution in Amherst, MA. I wanted to reach out to you and let you know how grateful I am for your service. From the bottom of my heart I want to thank you for defending our freedoms and way of life. Though decades have passed since you fought for our nation your sacrifice will never be forgotten.
Best,
Lord Jeff
A good letter will thank a soldier but a great letter will engage them on a deeper level. Do not just thank the soldier you are writing to; ask them questions about their experience overseas (i.e. what was the most beautiful place you visited), what it's like to serve in their branch (e.g. Army, Air Force, etc.), or what it's like to serve in their respective field (e.g. infantry, engineers, etc.).